This invention relates to a stack for polymer electrolyte fuel cells that use, as an electrolyte, a solid polymer having ion conductivity, and more particularly to a polymer electrolyte fuel cell stack improved to have a large electrode area.
Attention is now being paid to fuel cells that serve as highly efficient energy converting devices. These fuel cells are roughly divided into a low-temperature operable fuel cell such as an alkali fuel cell, a polymer electrolyte fuel cell, a phosphoric acid fuel cell, etc., and a high-temperature operable fuel cell such as a molten carbonate fuel cell, a solid oxide fuel cell, etc.
Among the above-mentioned fuel cells, a polymer electrolyte fuel cell (hereinafter referred to as a xe2x80x9cPEFCxe2x80x9d), which uses a solid polymer electrolyte membrane having proton conductivity, is expected to be used as a power supply for space or vehicle equipment, since it has a compact structure, provides a high output density and is operable by a simple system.
As the polymer electrolyte membrane (hereinafter referred to as a xe2x80x9cpolymer membranexe2x80x9d), a perfluorocarbon sulfonic acid membrane (e.g. Nafion: name of commodity, produced by Dupont Company), for example, is used. This polymer membrane is held between a pair of porous electrodes (an anode and a cathode) having a catalyst such as platinum, thereby constituting a membrane/electrode assembly. The polymer membrane and the porous electrodes are in the shape of a sheet, and have a thickness of about 1 mm or less so as to reduce the internal resistance thereof.
Further, the polymer membrane and electrode sheets are usually rectangular. The area of each electrode is determined on the basis of a current required for power generation and a current value per unit area, i.e. the current density. Most of the electrodes are set to have an area of about 100 cm2 or more, i.e. to have one side of 10 cm or more. The polymer membrane also has a function of preventing mixture of gasses supplied to the anode and the cathode, and hence is set to have a larger area than the electrodes.
To extract a current from the membrane/electrode assembly, current collectors are provided outside the anode and the cathode. The current collectors have a large number of grooves extending parallel to the surfaces of the anode and the cathode. These grooves serve as gas passages for supplying the anode and the cathode with a fuel gas and an oxidant gas required for reaction in the cell, respectively. Moreover, since voltage generated by a single membrane/electrode assembly is as small as 1V or less, a PEFC stack structure is formed by stacking a plurality of membrane/electrode assemblies and connecting them in series. This structure needs a cathode current collector and a cathode current collector, and therefore a separator is used, which includes collectors respectively provided at the anode side and the cathode side of the adjacent membrane/electrode assemblies, and formed integral as one body.
Each membrane/electrode assembly generates heat during reaction in the cell. It is a usually used cooling method to insert a cooling plate between a plurality of membrane/electrode assemblies and circulate cooling water in the cooling plate. This method, however, requires a separator for supplying the cooling water, in addition to a separator for supplying gases. This results in an increase in the thickness in the direction of stacking.
Japanese Patent Application KOKAI Publication No. 10-21949 discloses, as a method for solving the problem, a method for forming cooling water passages around the gas passages to dispense with the cooling plate inserted between the membrane/electrode assemblies. More specifically, in the technique disclosed in this publication, passages 202 for circulating cooling water are formed in upper, lower, left and right four portions of a separator 200 which has grooves 21 formed as gas passages at a central portion thereof, as is shown in FIG. 1, and cooling water is circulated in the passages 202 to eliminate reaction heat.
However, the above cooling method has the following problems:
A first problem is that the reaction area cannot be enlarged. In the above-mentioned cooling method, heat generated from the membrane/electrode assemblies that hold the separator 200 is transferred to the separator 200 conducted in a direction perpendicular to the thickness direction of the separator, and is removed by cooling water flowing through the passages 202. In other words, the temperature of a central reaction portion of the separator becomes higher than its peripheral portion.
Accordingly, if the reaction area is increased, the distance between the center of the reaction portion and each cooling passage increases, and a temperature difference, as above, also increases. On the other hand, increasing the thickness of the separator to thereby increase the cross section, i.e. the heat transfer area, can be contrived in order to reduce the temperature difference. This method, however, inevitably increases the thickness of the cell and hence the entire cell size.
A second problem is that a three-dimensional temperature distribution occurs in the separator plane. Specifically, in the above-described cooling method, the temperature is so distributed in the separator plane that it is higher at a central portion than at peripheral four sides. Therefore, even if the gas passages are formed flat, moisture created as a result of reaction condenses at a peripheral portion of the separator, and hence cannot be efficiently collected.
A third problem is that supply manifold and exhaust manifold for the fuel gas and the oxidant gas cannot be enlarged. Where cooling water passages are arranged around gas passages as shown in FIG. 21, the supply manifold and the exhaust manifold for the fuel gas and the oxidant gas must be arranged at the four corners, thereby reducing the cross sections of the supply manifold and the exhaust manifold than those of the cooling water passages.
This means that when the reaction area is enlarged and a great amount of fuel gas or oxidant gas is required, the fuel gas or the oxidant gas cannot uniformly be distributed to each cell of a fuel cell stack, since the cross section of the supply port, i.e. the cross section of a gas distributing manifold, inevitably reduces.
It is the object of the invention to provide a polymer electrolyte fuel cell stack, which is compact but has a large reaction area, and can smoothly supply gas.
To attain the object, there is provided a polymer electrolyte fuel cell stack including a plurality of cells stacked on each other, each cell having an anode, a cathode and a solid polymer electrolyte membrane held between the anode and the cathode, the cells being stacked on each other via separators that each have at least one of a fuel gas passage for supplying the anode with a fuel gas, and an oxidant gas passage for supplying the cathode with an oxidant gas, characterized in that: each of the separators has a rectangular outline; and a coolant passage is formed in a portion of each separator, which is located around the fuel gas passage and the oxidant gas passage and is substantially parallel to a long side of each separator, such that a coolant flows in a direction perpendicular to a surface of each separator.
Since in the invention constructed as above, each separator has a rectangular outline and has a coolant passage in a portion thereof substantially parallel to its long side, the distance between a central portion and the upper or the lower end of each electrode can be reduced. Accordingly, the temperature difference between the central portion and the upper or lower end of each electrode can be minimized. Further, heat generated during reaction is transferred vertically, and hence the temperature is almost constant horizontally. This means that even when the reaction area is enlarged, the temperature difference in each separator can be minimized. Furthermore, since it is not required to insert a cooling member in a direction in which cells are stacked, the thickness in the cell-stacked direction can be reduced.
According to another aspect of the invention, there is provided a polymer electrolyte fuel cell stack including a plurality of cells stacked on each other, each cell having an anode, a cathode and a solid polymer electrolyte membrane held between the anode and the cathode, the cells being stacked on each other via separators that each have at least one of a fuel gas passage for supplying the anode with a fuel gas, and an oxidant gas passage for supplying the cathode with an oxidant gas, characterized in that: each of the separator has a rectangular outline; and a plurality of coolant passages are formed in portions of each separator, which are located substantially parallel to opposite long sides of the separator, such that a coolant flows in a direction perpendicular to a surface of the separator.
Since in the invention constructed as above, a plurality of coolant passages are provided in portions of each separator substantially parallel to the opposite long sides of each separator, a more excellent cooling effect than that of the invention of claim 1 can be obtained.
According to yet another aspect of the invention, there is provided a polymer electrolyte fuel cell stack including a plurality of cells stacked on each other, each cell having an anode, a cathode and a solid polymer electrolyte membrane held between the anode and the cathode, the cells being stacked on each other via a separator that has at least one of a fuel gas passage for supplying the anode with a fuel gas, and an oxidant gas passage for supplying the cathode with an oxidant gas, characterized in that: each of the separators has a rectangular outline; and a surface of each separator, which contacts the electrodes, has a plurality of cooling areas, a coolant passage being formed in a central portion of each of the cooling areas such that a coolant flows in a direction perpendicular to a surface of each separator.
Since in the invention constructed as above, a coolant passage is formed in a central portion of each cooling area, reaction heat generated in each cooling area is removed by the coolant flowing through each cooling passage. In this case, the inner wall of each coolant passage serves as a heat transfer area. Since each coolant passage is situated at the center of each cooling area, its entire inner wall can be used as the heat transfer area. Accordingly, efficient cooling can be executed.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.